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. 2022 Oct 5;7(41):36468–36478. doi: 10.1021/acsomega.2c04279

Hydrogen-Rich and Clean Fuel Gas Production from Co-pyrolysis of Biomass and Plastic Blends with CaO Additive

Yuanjun Tang †,*, Jun Dong ‡, Yuan Zhao §, Guoneng Li †, Yong Chi ∥, Elsa Weiss-Hortala ⊥, Ange Nzihou ⊥, Guanqun Luo †, Chao Ye †
PMCID: PMC9583305  PMID: 36278101

Abstract

graphic file with name ao2c04279_0012.jpg

The treatment and disposal of waste biomass and plastics are of great importance to achieve both waste management and resource recycling. In this work, pyrolysis of biomass and plastic blends were investigated to identify the influence of temperature and in situ CaO addition on the production of hydrogen-rich, HCl-free, and low tar content fuel gases. The results show that the increase in temperature and the use of CaO significantly improved both the quantity and quality of the fuel gas and mitigated the formation of tar compounds and HCl. Moreover, H2 yield was significantly improved from 0.30 to 3.68 mmol/g with the increase in temperature from 550 to 850 °C. Also, the use of in situ CaO significantly increased the H2 yield by 28–88%. The H2/CO ratio was also enhanced from 0.35 to 1.50 with the temperature increase and CaO addition. Tar removal efficiency reached approximately 70.09% with the use of CaO at 850 °C. The produced HCl gas could be effectively absorbed by CaO through dechlorination reactions to form CaClOH at a highest mitigation efficiency of 92.37%. The results could be used to develop clean and efficient treatment technologies of waste biomass and plastics.

1. Introduction

The continuous increase in energy demand and the growing awareness about the environment have accelerated the growth of waste biomass and plastics management in China. Besides being treated as an environmental burden, waste biomass and plastics are also considered useful resources, supplying energy or serving as raw materials.1 Direct combustion of waste biomass and plastics is an effective method to realize waste disposal and energy recovery. Nevertheless, incineration of waste biomass or plastics has inevitable drawbacks of corrosion issues and toxic emissions, particularly acidic gases, dioxins, or heavy metals.2,3

Co-pyrolysis of waste biomass and plastics is among the potential alternatives to achieve both clean and efficient treatment and energy recovery.4−8 Instead of being all converted into thermal energy of flue gas, waste biomass and plastic blends are converted to value-added intermediates (such as fuel gas, solids, or oil) during pyrolysis.9−11 The intermediates can subsequently be stored, transported, or utilized in more environment-friendly and energy-efficient downstream pathways. Therefore, more research attention has been paid to pyrolysis-based technologies in recent years.4,12,13

Pyrolysis is widely recognized as the most primary step of waste thermal conversion and affects the yields and properties of the products in the initial stage. Therefore, the investigation of waste biomass and plastic blends pyrolysis is of great importance and meaningful to understanding the thermochemical conversion of combustible solid wastes. Co-pyrolysis of waste biomass and plastic blends has become a more and more popular research topic in the field of waste management. Publications focused on the pyrolytic performances that are affected by reactor types,14,15 reaction temperature,16,17 reaction pressure,18 residence time,19,20 and heating methods.21,22 Besides, co-pyrolysis of the blends offers a green and sustainable route for the generation of H2, which is considered the most important energy carrier for future societies.23,24 However, the formation of tar compounds and high CO2 content in the produced pyrolytic gases significantly limit the commercial applications of waste pyrolysis technologies.25 Moreover, the presence of chlorine in waste plastics (e.g., in PVC and rubber) would generate HCl in gaseous products, causing severe corrosion issues and accelerating the formation of toxic organic compounds. Therefore, the quality of produced fuel gas needs to be enhanced.

Catalysts or additives are commonly used to enhance the quality of pyrolytic gas by cracking tar compounds, absorbing CO2, and mitigating HCl contained in the produced fuel gases.26−30 Among the auxiliaries, calcium-based additives (e.g., calcium oxide, calcium hydroxide, and calcium carbonate) are the most preferable substances used to achieve the abovementioned aims. This is mainly due to the properties of low price and vast abundance on the earth of calcium-based additives. Among which, calcium oxide (CaO) is the most commonly used additives in many scenarios, and it plays important roles during waste biomass and plastic co-pyrolysis.31 On the one hand, CaO can be regarded as efficient CO2 absorbents and H2 production adjusters to enhance the quality of produced fuel gas by accelerating water–gas shift reactions.32 On the other, it can inhibit the formation of tar compounds in the produced fuel gas through tar catalytic thermal cracking.28,33,34 Furthermore, Ca-based additives are also used either directly in the furnace or in air pollution control systems for acid gas removal, such as the desulfurization process, which is employed in most of the coal-fired power plants. For the same reason, CaO can also serve as potential in situ reactants to mitigate HCl. The reason is because Ca-based additives can immobilize chlorine through heterogeneous gas–solid reactions to form solid-phase products, such as CaCl2 or CaClOH.

Accordingly, this study aimed to investigate the triple effects of reaction temperature and in situ CaO additive on: (1) the formation of H2-rich and low CO2 content fuel gas; (2) the reduction of tar compounds; and (3) the mitigation of HCl in the produced fuel gas during co-pyrolysis of waste biomass and plastic blends. It is expected that the knowledge obtained in this work can serve as a piece of fundamental understanding for the identification of more energy-efficient and environment-friendly thermal conversion techniques of waste biomass and plastics in China.

2. Materials and Methods

2.1. Waste Biomass and Plastic Blends and CaO Characteristics

To maintain the consistency of feedstock in the experiments, artificial waste biomass and plastic blends were used in all the experiments. It was prepared to match the typical waste biomass and plastic proportions in municipal solid waste in Hangzhou, China. The physical composition and the ultimate and proximate analyses results are presented in Table 1. Before the experiments, the blends were ground and sieved to particle sizes of below 1.00 mm. The CaO additive was prepared to size in the range of 0.25–0.43 mm and calcined at 850 °C for 2 h. The prepared CaO additive had the specific surface area, total pore volume, and average pore diameter of 2.37 m2/g, 0.039 cm3/g, and 25.0 nm, respectively.

Table 1. Composition and the Ultimate and Proximate Analyses Results for the Waste Biomass and Plastic Blends.

mass percentage of waste biomass and plastic blends
biomass (ara, wt %)
 plastics (ara, wt %)
flour paper wood polyethene (PE) polyvinylchloride (PVC)
55.0 15.0 5.0 15.0 10.0
mass percentage of waste biomass and plastic blends
proximate analysis (ada, wt %)
 ultimate analysis (ada, wt %)
moisture ash volatile fixed carbon C H Ob N S Cl
5.2 3.0 77.1 14.7 47.5 6.0 30.3 1.1 0.3 6.6
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a

ar means as received basis, and ad refers to air-dried basis.

b

By difference.

2.2. Test Facilities and Procedures

An experimental laboratory setup, mainly consisting of a tubular furnace unit, a gas source unit, and a sampling unit, was employed to study the influence of temperature and the addition of CaO on the thermal conversion behavior and HCl mitigation during the co-pyrolysis of waste biomass and plastic blends. The system is shown in Figure 1. High purity nitrogen gas with a flow rate of 120 mL/min was supplied as the carrier gas using a high-pressure gas cylinder. The tubular furnace reactor was electrically heated with an electrical power of 5.0 kW. A K-type thermocouple was inserted in the middle of the heating area to control and record the temperature profile. The reaction zoom was made of a quartz tube with a length of 700.0 mm and a diameter of 40.0 mm. A water-cooled condenser was used after the reactor to collect the liquid condensates. Three ice-bath-cooled impingers were used to collect HCl gas and to remove moisture (water). The first two impingers contained 0.1 mol dm–3 sodium hydroxide solution, whereas the third one contained silica gel for moisture removal.

Figure 1.

Figure 1

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Schematic diagram of the experimental facility: 1, high-pressure gas cylinder; 2, flow meter; 3, reactor; 4, quartz boat; 5, controller; 6, condenser; 7, cooling water; 8, impingers with 0.1 mol dm–3 NaOH solution; 9, impinger with silica gel; 10, ice-bath cooler; and 11, micro-GC gas analyzer.

Before the experiments, the quartz tubular furnace was preheated to the desired temperature via electrical heating. Once the temperature stabilized, the quartz boat, with 5.0 g of biomass and plastic blends and CaO additive (0.26 g for Ca/Cl molar ratio of 1:2; 0.52 g for 1:1; 1.04 g for 2:1; and 2.08 g for 4:1) was uniformly mixed together and then pushed into the center of the reactor quickly to avoid air leakage. The reaction time (30 min) was the same for all the experiments. Solid residues and liquid products were accurately measured by weighing the quartz boat and the condenser, respectively. HCl gas was absorbed by the solutions through the gas trapping route.

2.3. Analysis Method

The composition of the produced gas was analyzed using a micro gas chromatograph analyzer (micro gas chromatograph 490 analyzer, Agilent). The produced fuel gas contained H2, CO, CH4, C2Hm (the sum of C2H2, C2H4, and C2H6), CO2, and N2. The analyzer was equipped with 10 m MS5A, 10 m PPU, and 8 m 5CB (columns) with helium as the carrier gas. The total gas yield was quantified based on a N2 balance by comparing the N2 concentration in inlet and production gases. The concentration of HCl gas was detected using an ion chromatograph (Thermo Fisher Scientific, Integration). Liquid products were collected from the condenser and by washing the reactor walls and connecting pipes. Tar composition was identified using gas chromatography–mass spectrometry (Agilent 8860-5977B). Solid products were collected from the quartz boat after each experiment. Also, they were detected using X-ray diffraction (XRD, D8 Advance Bruker) and SEM (Carl Zeiss, Ultra 55). For experiments with CaO addition, the mass of additionally added CaO was subtracted from the solid products.

To evaluate the quality of the produced fuel gas, the carbon conversion efficiency (ηC) is determined using eq 1 as

2.3. 1

where yCO, yCO2, yCH4, and yC2Hm are the molecular fractions of CO, CO2, CH4, and C2Hm in the produced fuel gas.

The effect of the increase in temperature and the use of in situ CaO additive on the removal efficiency of condensate compounds were measured by thermal removal efficiency of tar (TtRE) and catalytic removal efficiency of tar (TcRE), which are given by eqs 2 and 3, respectively.

2.3. 2
2.3. 3

The effect of CaO on the mitigation of HCl was determined using the mitigation efficiency of HCl (HME) by eq 4

2.3. 4

To make sure of the reliability and reproducibility of the experimental results, all the experiments were repeated thrice, and the results reported in the present work are the average values. The mass balance of the solids, liquids, and gases reached 85–96% for all the experimental runs.

3. Results and Discussion

3.1. Mass Distribution of Pyrolytic Products

Figure 2 shows the increase in temperature and the use of in situ CaO additive on the mass distribution of pyrolysis products. Temperature is one of the determinant factors affecting the pyrolytic reactions. It significantly influences the yields of the produced gas, solid residues, and liquid products from the co-pyrolysis of waste biomass and plastic blends. The results showed that higher temperatures improved the generation of produced gas (continuously increased from 32.8 to 68.6% within the experimental temperature range of 550–850 °C) and simultaneously decreased the yield of solid residues and liquid products. The mass percentage of solids declined slightly from 20.4 to 12.6%, whereas the liquid yields sequentially reduced from 46.8 to 18.6% with increased temperature. The results were in good agreement with previous findings.35−37 It is well recognized that changing the temperature will significantly affect the chemical reaction results due to the shift of the thermodynamic equilibria. For the co-pyrolysis of the blends, higher temperatures favor the devolatilization at the initial stage of the reactions, as shown by eq 5. Meanwhile, higher temperatures also significantly accelerated the thermal cracking reactions of condensate liquids to form incondensable light hydrocarbons or gases (as given by eq 6).

3.1. 5
3.1. 6

Figure 2.

Figure 2

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Effect of the increase in temperature and the presence of in situ CaO additive on mass distribution of pyrolysis products (Ca/Cl molar ratio = 1:1).

Concerning the influence of CaO additive, the use of CaO decreased the yields of liquid product by around 4.6–15.4% within the studied experimental temperature, but increased the generation of both the solid residues (by 3.6–5.8%) and produced fuel gas (by −0.4 to 11.8%). Similar results were reported in the literature, indicating that the addition of in situ CaO during the pyrolysis of waste biomass and plastics significantly enhances the catalytic thermal cracking reactions of tar compounds, leading to the conversion of liquid condensates to incondensable gases and light hydrocarbons, as shown by eq 7.26,28,31,38 Regarding the solid products, due to the presence of CaO additive, some acidic gases, especially CO2 and HCl were immobilized in the solid residues and thus the yield increased by a slight bit.

3.1. 7

3.2. Generation of Hydrogen-Rich Fuel Gas

3.2.1. Combustible Fuel Gas Properties

Figure 3 shows the effect of temperature and CaO additive on the yield of total fuel gas and the combustible gas composition (i.e., H2, CH4, CO, and C2Hx). The results show that, with the increase in temperature from 550 to 850 °C, the yield of total fuel gas improved significantly from 81.9 mL/g-blends to 451.4 mL/g-blends. Meanwhile, the molecular fraction of H2 and CH4 continuously increased from 8.3 to 18.3% and from 13.7 to 31.8%, respectively. It is mainly due to the enhancement of endothermic devolatilization and the degradation of condensates (as presented in eqs 5 and 6) at higher temperatures to generate more combustible gases. The content of carbon monoxide (CO) in the fuel gas fluctuated within the range of 16.7–23.6% under the experimental temperatures. The reason is mainly attributed to the competing reactions among the degradation of condensates (eq 6), the water–gas reaction (eq 8), and the water–gas shift reaction (eq 9) to either generate CO or consume CO.39 Similar findings were reported in the literature.40 Regarding C2Hm (the sum of C2H2, C2H4, and C2H6), the fraction of C2Hm increased rapidly from 8.8 to 24.7% when the experimental temperature increased from 550 to 650 °C. However, when the temperature is further increased, the content of C2Hm reduced to 19.1% at 850 °C. The increase of C2Hm fraction at the initial stage was mainly due to the acceleration of condensates’ thermal cracking to generate more light hydrocarbons with the increase in temperature (eq 6). However, a further increase in the temperature favored the reforming reactions of hydrocarbons (as shown by eq 10), which resulted in the consumption of C2Hm at higher temperatures.

3.2.1. 8
3.2.1. 9
3.2.1. 10
Figure 3.

Figure 3

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Effect of the increase in temperature and the use of in situ CaO on the composition of produced combustible gases and the yield of total fuel gas (Ca/Cl molar ratio = 1:1).

The influence of in situ CaO additive on the production of fuel gas and the composition of combustible gases were also investigated, and the corresponding results are shown in Figure 3. The yield of the total fuel gas ranged from 140.7 to 496.74 mL/g-blends with the presence of in situ CaO at the studied temperature range and increased by 10.1–71.9% as compared to the conditions without CaO. Furthermore, the content of H2 gas was promoted by adding CaO, though the generation of CO was inhibited. This can be attributed to the acceleration of the water–gas shift reaction (as shown by eq 9). With the addition of CaO, the produced CO2 was absorbed through the carbonation reaction to form CaCO3. The consumption of CO2 would thus lead to the acceleration of eq 9 to produce more H2 and consume more CO. This fact was confirmed by the XRD results as presented in Figure 4, which revealed the formation of CaCO3 from CaO with the produced CO2 gas. The results show that more CaCO3 was formed at lower experimental temperatures. When the temperature was increased, the intensity of XRD results declined sharply. This is connected with the thermal decomposition of the produced CaCO3 at higher temperatures, which will result in the decomposition of the produced CaCO3.

Figure 4.

Figure 4

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Effect of the increase in temperature and the use of in situ CaO on solid products.

Regarding light hydrocarbons (CH4 and C2Hm), the content increased dramatically by approximately 21.7% with CaO addition at a low temperature of 550 °C. This reason is mainly attributed to the enhancement of catalytic cracking of the liquid condensates (eq 6). However, at higher temperatures of 650–850 °C, the molar fraction of light hydrocarbons reduced by around 1.6–2.1% with the addition of CaO, the reason is due to the strength of the reforming reactions of hydrocarbons by CaO (eq 10).

3.2.2. Carbon Conversion Efficiencies

Carbon conversion efficiency reflects the molecular fraction of carbon originating from the feed and getting converted into the fuel gas as expressed in eq 1. As demonstrated in Figure 5, the increase in reaction temperature and CaO favored the transfer of carbon from the waste biomass and plastic blends to the produced gas. Carbon conversion efficiency was increased from 10.11% under the condition of 550 °C without CaO to 60.13% under the condition of 850 °C with CaO additive.

Figure 5.

Figure 5

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Carbon conversion efficiency from the feedstock to the produced fuel gas.

3.2.3. Hydrogen Yield and H2/CO Ratio

The production of H2 gas and the molecular ratio of H2/CO are vital indicators to demonstrate the quality of fuel gas and determine directly its further applications. The more the H2 was obtained, the higher the fuel gas quality could be expected. It is reported that, with the increase in the H2/CO ratio, the produced fuel gas could be utilized in Fischer–Tropsch process (with Fe- and Co-based catalyst) (H2/CO within the range of 0.5–1.5), Fischer–Tropsch synthesis (H2/CO within the range of 1.5–3.0), gas turbine combustion (H2/CO within the range of 2.5–4.0), or solid oxide fuel cells (H2/CO within the range of 4.0–6.0).41,42 Therefore, the variations in the yield of H2 and the ratio of H2/CO as functions of temperature and CaO additive are presented in Table 2. When the pyrolysis temperature varied from 550 to 850 °C, the production of H2 improved dramatically from 0.30 to 3.68 mmol/g-blends. Meanwhile, the molecular ratio of H2/CO increased from 0.35 to 1.09. The reason is mainly attributed to the enhancement of the devolatilization of waste biomass and plastics and also the degradation of condensates to generate more H2 under higher temperatures.

Table 2. Effect of the Increase in Temperature and the Use of In situ CaO on the Yield of H2 Gas and the Molecular Ratio of H2/CO.
  without CaO
 with CaO (Ca/Cl = 1:1)
  H2 yield (mmol/g-blends) H2/CO ratio H2 yield (mmol/g-blends) H2/CO ratio
550 °C 0.30 0.35 0.57 0.59
650 °C 0.76 0.53 1.42 1.03
750 °C 2.27 0.90 2.92 1.05
850 °C 3.68 1.09 4.72 1.50
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With regards to CaO additive, the use of in situ CaO significantly enhanced the yield of H2 (from 0.57 to 4.72 mmol/g-blends) as well as the H2/CO ratio (from 0.59 to 1.50), revealing a significant enhancement in the produced fuel gas quality through the addition of in situ CaO. The acceleration of the water–gas shift reaction is responsible for this phenomenon. Furthermore, it is also worth mentioning that, under 850 °C and with CaO addition, the generated syngas could directly be used in Fischer–Tropsch synthesis because a remarkable higher H2/CO ratio of 1.5 was obtained under such circumstances.

3.3. Generation of Clean Fuel Gas

3.3.1. Tar Compound Mitigation

One of the most serious bottlenecks in the utilization of pyrolytic fuel gas is the formation of tar compounds (i.e., the liquid condensates). Tar compounds may not only block or damage the devices but also limit further utilization of the products. Therefore, the yield and thermal/catalytic thermal removal efficiency of tar were determined and the corresponding results are presented in Figure 6. The results show that the increase in temperature and the use of in situ CaO additive sharply reduced the formation of condensates in the products during the pyrolysis of waste biomass and plastic blends. It is well known that the decomposition of tar occurs through two pathways: one is thermal cracking (eq 6), and the other is catalytic cracking (eq 7). As shown in Figure 6a, the content of tar declined dramatically from 0.47 g/g-blends at 550 °C to 0.19 g/g-blends at 850 °C without CaO additive. This is mainly attributed to the increase in temperature, which accelerated the degradation of condensates to light hydrocarbons. With the presence of CaO, the content of tar further decreased from 0.38 to 0.14 g/g-blends when the temperature was varied in the range of 550–850 °C. This is mainly due to the presence of O2– ions on the active sites of CaO’s surface, which destabilized the spatial stability of π-electron clouds of the aromatic ring structures and promoted the ring-opening reactions of aromatic compounds, thus achieving the catalytic degradation of tar.38,43,44

Figure 6.

Figure 6

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Tar yields and tar removal efficiencies under the experimental conditions (Ca/Cl molar ratio = 1:1): (a) tar yield and (b) tar removal efficiency.

Figure 6b reports the thermal and catalytic removal efficiencies of tar compounds during the pyrolysis of the blends. According to eqs 2 and 3, tar yield at 550 °C without CaO addition was set as the baseline. The results show that tar thermal removal efficiency (TtRE) continuously increased from 25.2 to 60.3% with the increase in pyrolysis temperature from 650 to 850 °C, which means more than half of the generated tar under low experimental temperature (baseline) would be decomposed at a higher temperature of 850 °C. The in situ CaO addition enhanced tar removal efficiency by promoting catalytic cracking (eq 7). Compared to the thermal removal efficiency of tar, a 9.8–32.9% increment in tar catalytic thermal removal efficiency (TcRE) was obtained. It is worth mentioning that the effect of CaO on the catalytic thermal cracking of tar decreased gradually with the increase in temperature. Within the temperature range of 550–750 °C, the catalytic removal efficiency of tar increased significantly from 19.2 to 67.5%. However, when the temperature was further increased to 850 °C, the increase in TcRE was limited only at around 2.5%. This was because under higher experimental temperature, a large portion of tar compounds had already decomposed due to thermal cracking, so the effect of CaO as a catalyst for catalytic cracking was not apparent. Meanwhile, CaO properties (pore structure and specific surface area) at higher temperatures might have degraded due to sintering and carbon deposition at higher temperatures.

Figures 7 and 8 show the characterization results of the tar compounds at temperature of 550 and 750 °C with/without CaO addition. Results reveal that at lower pyrolysis temperature of 550 °C, the main pyrolytic tar compounds were chain hydrocarbons. The reason is mainly due to the content of plastics in the blends (15% PE and 10% PVC). Plastics are easily decomposed at low temperatures and generate a large number of chain hydrocarbons, especially olefins, due to the fracture of polymer carbon chains. However, at a higher pyrolysis temperature of 750 °C, the main compounds changed to aromatic hydrocarbons, and no chain hydrocarbons were detected. The reasons are mainly composed of two aspects: on the one hand, higher temperature promotes the decomposition and reforming of aliphatic chain hydrocarbons to form light gases, such as CH4, H2, CO, and CO2 and on the other hand, at higher temperatures, chain hydrocarbons undergo dehydrogenation and cyclization to form aromatic compounds.

Figure 7.

Figure 7

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Characterization of tar compounds at 550 °C.

Figure 8.

Figure 8

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Characterization of tar compounds at 750 °C with CaO (Ca/Cl molar ratio = 1:1) and without CaO.

3.3.2. HCl-Free Gas Production

Beside tar compounds, Cl-containing contaminants are also key obstacles for the clean and efficient utilization of pyrolytic fuel gases, especially for waste plastics with high chlorine content (e.g., PVC). The use of in situ CaO could react with HCl gas to generate CaClOH (see XRD spectra shown in Figure 4), thus preventing the release of chlorine into the produced fuel gas. Therefore, the effect of temperature and the use of in situ CaO on the mitigation of HCl gas were also considered in the present work.

The HCl mitigation efficiency as a function of temperature and Ca/Cl molecular ratio was investigated and the corresponding results are illustrated in Figure 9. The results show that the variation of temperature from 550 to 850 °C had an adverse impact on the mitigation of HCl gas by CaO addition. For the molecular ratio of Ca/Cl at 2/1, the value of the mitigation efficiency of HCl (HME) decreased continuously from 92.4 to 65.2%. It is worth mentioning that the decrease of HME value was accelerated at higher temperatures. HME value decreased by 8.8% in the temperature range of 550–750 °C; whereas, it reduced sharply by 18.3% when the temperature was further increased to 850 °C. This can be explained from two aspects. On the one hand, according to Shemwell et al.,45 the binding energy of the in situ CaO additive and the produced HCl gas was decreased at higher temperatures because of the variation of the chemical equilibrium constant of dechlorination reactions (eqs 11–14),46 therefore the dechlorination reactions were limited at higher temperatures. On the other hand, at a temperature higher than 750 °C, surface melting of dechlorination products occurred (i.e., the melting temperature of CaCl2 was 772 °C47).

3.3.2. 11
3.3.2. 12
3.3.2. 13
3.3.2. 14
Figure 9.

Figure 9

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Effect of the increase in temperature and the molecular ratio of Ca/Cl on the mitigation efficiency of HCl gas.

Regarding the influence of the molecular ratio of Ca/Cl, the stoichiometric molecular ratio of Ca/Cl should be 1/2 or 1/1, depending on the final products CaCl2 or CaClOH, respectively. However, the HME value under such conditions was relatively low and ranged only from 42.2 to 69.6%. This means that the overuse of CaO additive was essential to achieve acceptable HME levels. Meanwhile, at 750 °C, the HME value increased dramatically by around 41.2% when the molecular ratio of Ca/Cl was increased from 1/2 to 2/1. However, further increase in the molecular ratio to 4/1 resulted in a limited increment of HME by approximately 4.8%. Therefore, in this work, the molecular ratio of Ca/Cl at 2/1 was considered the most appropriate proportion of in situ CaO to mitigate HCl. The optimal working condition was obtained at 750 °C (from the gas quality perspective) and the molecular ratio of Ca/Cl at 2/1 (from the HME perspective). In this case, the HCl gas mitigation efficiency reached 83.6%. It also has to be clarified that the amount of CaO added into the tubular furnace did not utterly equal the amount of the CaO reacted. Therefore, in this work, the molar ratio, as well as the stoichiometric amount of CaO indicates only the amount of CaO added to the system.

The results from XRD spectra further confirmed the dechlorination results. As shown in Figure 4, the obtained minerals mainly included CaCO3, CaO, and CaClOH. The only Cl-containing solid product detected was CaClOH, which means eqs 11 and 13 were the main dechlorination reactions in the reactor in this work. Similar conclusions were demonstrated by Li,27 who reported that CaClOH and CaCO3 were formed during the adsorption of CO2/HCl mixtures by CaO reactant. Xie48 had concluded that both CaClOH and CaCl2 were dechlorination products by Ca-based additives. CaClOH was generated for a short dechlorination time (at dozens of minutes), whereas CaCl2 was generated when the reaction time was long enough (at dozens of hours). Therefore, CaClOH has recorded as the main dechlorination product during the co-pyrolysis of waste biomass and plastic blends with the use of in situ CaO in this work.

Figure 10 shows the SEM characterization results of the solid productions. As presented, the surface of the solid products at the reaction temperature of 550 °C was looser and more porous compared to that at higher temperatures. With the increase in reaction temperature, the surface became denser and denser. At 750 °C, it was obviously melted and smoothed, indicating that the sintering and melting of the dechlorination products occurred at the reaction surface. The reason is, as aforementioned, mainly because the melting temperature of the dechlorination product, CaCl2, was 772 °C. The SEM results confirmed that calcination and sintering of the reactants and the product layers at higher temperatures changed the surface properties of CaO, thus inhibiting the mitigation of HCl by CaO additive.

Figure 10.

Figure 10

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SEM images of solid products at experimental temperatures: (a) 550; (b) 650; (c) 750; and (d) 850 °C.

4. Conclusions

In this work, the effect of temperature and CaO addition on the co-pyrolysis of waste biomass and plastics were experimentally investigated in a fixed-bed tubular furnace reactor. Yields and properties of the produced fuel gas, tar cracking, and the mitigation efficiency of HCl gas by in situ CaO additive were determined. The results show that the temperature was the most critical factor determining the yields of three-phase products. Higher temperatures promoted the yield of fuel gas and reduced the production of condensate liquids and solid residues. With the increase in experimental temperature from 550 to 850 °C, the yield of fuel gas increased sharply by approximately 5.5 times from 81.9 to 451.4 mL/g-blends. With the addition of CaO, the total gas yield increased by 30.0–58.8 mL/g-blends. The content of tar was significantly reduced from 0.47 g/g-blends without CaO at 550 °C to 0.14 g/g-blends in the presence of CaO at 850 °C by thermal cracking and catalytic cracking of tar. Moreover, HCl was also immobilized into the solid residues by dechlorination reactions with the addition of CaO to generate CaClOH. From both the gas quality and HCl mitigation perspectives, the pyrolysis temperature of 750 °C and the Ca/Cl molecular ratio of 2/1 was found to be the optimum reaction condition with the HCl mitigation efficiency of 83.6%.

Acknowledgments

This project was supported by the Zhejiang Provincial Natural Science Foundation of China (grant nos. LQ21E060001, LZ21E060001, LQ21E060002) and the National Natural Science Foundation of China (grant nos. 12202390, 52006196).

The authors declare no competing financial interest.

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